This invention relates to improvements in the stabilization of perchloromethyl mercaptan. More particularly, it relates to the use of phosphorus derivatives as stabilizing agents.
Perchloromethyl mercaptan, Cl.sub.3 CSCl, also known as trichloromethanesulfenyl chloride, has commercial importance as an intermediate in the manufacture of fungicides, bactericides, germicides, herbicides, soil fumigants and pharmaceuticals.
Perchloromethyl mercaptan was first described in a production scheme by Rathke in Annalen, Volume 167, at page 195 (1873). Rathke's method, which is still in use today, utilizes an iodine catalyst. The reaction scheme operates most efficiently at temperatures below about 40.degree. C., in accordance with the following equations: EQU CS.sub.2 + 3Cl.sub.2 .fwdarw. CCl.sub.3 SCl + SCL.sub.2 (1) EQU 2cs.sub.2 + 5cl.sub.2 .fwdarw. 2CCl.sub.3 SCl + S.sub.2 Cl.sub.2 (2) EQU CS.sub.2 + 3Cl.sub.2 .fwdarw. CCl.sub.4 + .sub.2 Cl.sub.2 (3)
In addition to sulfur dichloride, sulfur chloride (also known as sulfur monochloride) and carbon tetrachloride, the reaction can also form thiophosgene and other compounds as unwanted byproducts. Although more volatile byproducts such as carbon tetrachloride and sulfur dichloride can be removed from the reaction mixture by distillation, it is extremely difficult to separate perchloromethyl mercaptan from sulfur chloride by this method. This is due to the fact that the boiling points of perchloromethyl mercaptan and sulfur chloride are very close to each other.
The prior art has proposed several methods for improving the basic Rathke method. For example, U.S. Pat. No. 3,544,625 to Masat, discloses a method for producing perchloromethyl mercaptan by chlorinating carbon disulfide in the presence of a solution of inorganic acids, such as hydrochloric acid. U.S. Pat. No. 3,673,246 to Meyer et al, discloses a continuous process for producing perchloromethyl mercaptan wherein carbon disulfide is reacted with chlorine on or in intimate contact with activated carbon at temperatures of about -5.degree. C. to +100.degree. C. U.S. Pat. No. 3,808,270 to Rupp et al discloses a continuous process for producing perchloromethyl mercaptan by reacting carbon disulfide and chlorine in a reaction zone filled with granular active carbon completely immersed in the liquid reaction mixture while maintaining temperatures in the range of about 40.degree. C. to about 135.degree. C. U.S. Pat. No. 3,878,243 to Zupancic discloses a homogeneous catalyst system comprising a lead salt of a carboxylic acid which is soluble in carbon disulfide. Notwithstanding, the proliferation of technology dealing with approaches other than the iodine catalyzed method, the basic Rathke method is still the predominant approach used to produce perchloromethyl mercaptan in the United States and abroad.
In the production of perchloromethyl mercaptan via the iodine catalyzed method, the crude perchloromethyl mercaptan must be separated from the reaction byproducts and the iodine catalyst by distillation. Since perchloromethyl mercaptan produced by the iodine catalyzed method is accomplished in a batch operation, some lapse of time occurs before the crude perchloromethyl mercaptan can be purified by distillation. One problem that occurs, is that the crude perchloromethyl mercaptan and its byproducts will react in the following manner: ##STR1##
The above reactions are undesirable because they produce S.sub.2 Cl.sub.2, a byproduct which is difficult to separate from perchloromethyl mercaptan by distillation, thereby reducing the purity of the final product. Also, the Cl.sub.2 produced in reaction (4) can react with perchloromethyl mercaptan as in reaction (6) to further reduce the yield of perchloromethyl mercaptan. Reactions (5) and (6) can also convert perchloromethyl mercaptan to CCl.sub.4, further reducing the yield of perchloromethyl mercaptan.
U.S. Pat. No. 3,479,253 discloses stabilizers for the production of sulfur dichloride such as trialkyl phosphites, phosphorus pentachloride or trialkyl phosphates, to be used in the distillation of sulfur dichloride to inhibit the decomposition of sulfur dichloride into sulfur monochloride and chlorine.
The present invention has achieved improvements in the production of perchloromethyl mercaptan via the use of phosphorus derivatives as stabilizers which are believed to suppress the occurrence of the undesirable reactions occurring in equations 4, 5 and 6.